This invention relates to highly selective, heterogeneous catalysts for the production of monoalkylene glycol. The invention also relates to a method of preparing such catalysts and a process for producing monoalkylene glycol using such catalysts.
Commercial processes for preparing alkylene glycols, for example ethylene glycol, propylene glycol and butylene glycol, involve liquid-phase hydrolysis of the corresponding alkylene oxide in the presence of a large molar excess of water (see, for example, Kirk-Othmer, Encyclopedia of Chemical Technology, Vol. 11, Third Edition, page 939 (1980)). The hydrolysis reaction typically is conducted at moderate temperatures, e.g., from about 100.degree. C. to about 200.degree. C., and elevated pressures. Water typically is provided to the reaction zone in excess of 15 moles per mole of alkylene oxide. The primary by-products of the hydrolysis reaction include di- and polyglycols, e.g., dialkylene glycol, trialkylene glycol and tetraalkylene glycol. The di- and polyglycols are believed to be formed primarily by reaction of alkylene oxide with alkylene glycol, since alkylene oxide is generally more reactive with alkylene glycol than water. The large excess of water therefore is employed in order to favor the reaction with water and obtain a commercially attractive selectivity to monoalkylene glycol.
Due to the large excess of water used in conventional processes, recovery of alkylene glycol from the hydrolysis reaction mixture is very energy intensive. Water is usually removed from the product stream by evaporation, and the remaining alkylene glycol-containing residue is purified further by distillation. A process which would permit a reduction in the amount of water employed while maintaining or enhancing selectivity toward monoalkylene glycol would reduce energy expenses as well as capital costs.
Many types of soluble or homogeneous catalysts have been proposed for this purpose. For example, U.S. Pat. No. 4,277,632 discloses a process for producing alkylene glycol by hydrolysis of alkylene oxide in the presence of a catalyst containing molybdenum or tungsten. The catalyst may be metallic molybdenum or metallic tungsten, or inorganic or organic compounds thereof, such as oxides, acids, halides, phosphorous compounds, polyacids, alkali and alkaline earth metal compounds, ammonium salts, and heavy metal salts of acids and organic acid salts. Hydrolysis of alkylene oxide takes place in the presence of about one to five times the stoichiometric amount of water without forming appreciable amounts of by-products such as polyglycols. The reaction may be carried out in the presence of carbon dioxide.
Japanese Kokai No. 54/128,507 discloses a process for producing alkylene glycol from alkylene oxide and water using metallic tungsten or tungsten compounds.
Japanese Kokai No. 56/073,035 discloses a process for the hydrolysis of alkylene oxide in the presence of carbon dioxide and a catalyst consisting of a compound containing at least one element selected from the group of titanium, zirconium, vanadium, niobium, tantalum and chromium The compounds include the oxides, sulfides, acids, halides, phosphorous compounds, polyacids, alkali metal salts of acids and polyacids, ammonium salts of acids and polyacids, and heavy metal salts of acids.
Japanese Kokai No. 56/073,036 discloses a process for the hydrolysis of alkylene oxide in the presence of carbon dioxide and a catalyst consisting of a compound containing at least one element selected from aluminum, silicon, germanium, tin, lead, iron, cobalt and nickel.
Japanese Kokai No. 56/92228 is directed to processes for producing highly pure alkylene glycol. A distillation procedure for recovering a molybdenum or tungsten-containing catalyst from an alkylene oxide hydrolysis product in the presence of carbon dioxide is disclosed. The catalyst comprises at least one compound selected from the group consisting of compounds of molybdenum and tungsten, which compound may be in combination with at least one additive selected from the group consisting of compounds of alkali metals, compounds of alkaline earth metals, quaternary ammonium salts and quaternary phosphonium salts. The preferred catalysts are molybdic acid, sodium molybdate, potassium molybdate, tungstic acid, sodium tungstate and potassium tungstate. Potassium iodide is the only additive employed in the examples. For a similar disclosure see, Japanese Kokai No. 56/90029.
U.S. Pat. No. 4,551,566 discloses the production of monoalkylene glycols with high selectivity by reacting a vicinal alkylene oxide with water in the presence of a water-soluble vanadate. Lower water to alkylene oxide ratios can be employed using this process, which results in attractive selectivities to monoalkylene glycol. The counter ion to vanadate is selected to provide a water-soluble vanadate salt under the reaction conditions employed, and alkali metals, alkaline earth metals, quaternary ammonium, ammonium, copper, zinc, and iron are suggested cations. U.S. Pat. No. 4,551,566 also discloses that vanadate may be introduced into the reaction system as a salt or on a support such as silica, alumina, zeolites or clay. Since the vanadate ion is water-soluble, it can be lost from the reaction system. Therefore means must be provided to recover it from the reaction effluent.
In U.S. Pat. No. 4,578,524, the reaction of alkylene oxide and water to form monoalkylene glycol is carried out in the presence of a diassociatable vanadate salt and carbon dioxide.
Although homogeneous catalysts comprising water-soluble salts of vanadate, molybdate, tungstate and other metalates have provided reasonable selectivity to monoalkylene glycol, they are difficult to recover from the hydrolysis products. Accordingly, attention has been directed to the insoluble salts of metalates, which, though more easily recoverable from solutions, traditionally have provided lower selectivities to monoalkylene glycol.
U.S. Pat. No. 4,667,045 discloses the production of alkylene glycol with high selectivity from alkylene oxide and water in the presence of organosalts of a metalate anion having at least one cyclic alkylenedioxy moiety. Particularly preferred metals for the metalate anions are vanadium, molybdenum and tungsten.
European Patent Publication 160,330 describes a process for making alkylene glycols from alkylene oxide and water in the presence of a metalate anion associated with an electropositive complexing site on a solid support, such as an anion exchange resin. Again, metalate anions of the metals vanadium, molybdenum and tungsten are preferred.
Clay minerals belonging to the hydrotalcite-sjogrenite-pyroaurite class have been of interest in preparing insoluble alkylene glycol catalysts. For a general description of these materials, see Reichle, "Catalytic Reactions by Thermally Activated, Synthetic, Anionic Clay Minerals", Journal of Catalysis, 94: 547-557 (1985). These clay minerals are composed of infinite layers of metal or nonmetal oxides and hydroxides stacked on top of each other. In cationic clays, these layers are negatively charged and interlayer cations rest in between the layers to neutralize the structure. Anionic clays have positively charged metal oxide/hydroxide layers with anions located interstitially. Many anionic clays contain hydroxides of both main group metals (i.e., Mg, Zn, Al) and transition metals (i.e., Fe, Co, Ni, Cr). The structure of these clays is similar to brucite, Mg(OH).sub.2, in which magnesium ions are octahedrally surrounded by hydroxyl groups with the resulting octahedra sharing edges to form infinite sheets. Some of the magnesium is isomorphously replaced by a trivalent ion, for example Al.sub.3+. The Mg.sub.2+ / Al.sub.3+ / OH.sup.- layers are then positively charged, and insertion of anions into anionic sites between the layers renders the overall structure electrically neutral.
One naturally-occurring, anionic clay is hydrotalcite, in which carbonate ion is present in the interstitial anionic sites. Hydrotalcite has the idealized unit cell formula Mg.sub.6 Al.sub.2 (OH).sub.16 -CO.sub.3.4H.sub.2 O. However, the ratio of Mg to Al in hydrotalcite can vary between 1.7 and 4, and various other divalent and trivalent cations may be substituted for magnesium and aluminum. In addition, the anion may be other than carbonate, and can be changed by synthesis, ion exchange or neutralization.
Hydrotalcite-type compounds have been used in many applications. EPO 207 707 A2 and U.S. Pat. No. 4,454,244 to Woltermann relate to the use of hydrotalcites as ion exchange materials. Schaper, "Stabilized Magnesia: A Novel Catalyst (Support) Material", Applied Catalysis, 54: 79-90 (1989) describes the use of hydrotalcites in the double-bond isomerization of 1-pentene. U.S. Pat. No. 4,883,533 to Kosin et al. discusses phosphate-containing hydrotalcites for improving the flame retardant characteristics of plastics and elastomers. JP 80/64,525 relates to the hydrolysis of alkylene carbonates using hydrotalcites. U.S. Pat. No. 4,530,918 to Sambrook et al. describes the use of nickel/aluminum/lanthanum hydrotalcites for use in steam reforming of hydrocarbons.
U.S. Pat. Nos. 3,796,792; 3,879,523 and 3,879,525 to Miyata et al. describe hydrotalcites having both cationic and anionic substitutions. The anionic substitutions may involve CrO.sub.4.sup.2-, MoO.sub.4.sup.2- and M.sub.2 O.sub.7.sup.2-.
U.S. Pat. Nos. 4,458,026 and 4,476,324 to Reichle describe heat treated hydrotalcite for conversion of acetone to mesityl oxide and isophorone and the aldol condensation of other carbonyl-containing compounds. These patents disclose the use of long chain aliphatic alpha-omega dicarboxylates, such as adipic, decane and dodecane dicarboxylates, as interstitial anions.
Reichle, "Catalytic Reactions by Thermally Activated, Synthetic, Anionic Clay Materials", Journal of Catalysis, 94:547-557 (1985) describes the effect of varying certain characteristics of hydrotalcite-type materials, and the use of these materials in vapor-phase aldol condensations, acetone oligomerization, and hydrogen-deuterium exchange. Reichle states that the type of anion in the interstitial sites in the hydrotalcite material affects the conversion of acetone to mesityl oxide and isophorone, and the size of the interstitial anion affects the spacing between layers.
Miyata et al., "Synthesis of New Hydrotalcite-like Compounds and Their Physico-chemical Properties", Chemistry Letters, pp. 843-848 (1973) relates to hydrotalcites for anion exchange, molecular sieving, and air separation. Miyata et al. describe the relationship between spacing of layers and such characteristics as dehydration temperature, carbon dioxide adsorption and number of carbon atoms in the interstitial anion.
Japanese Kokai No. 55/69525 and Japanese Kokai No. 57/106631 disclose using hydrotalcite and its analogs wherein magnesium is replaced by calcium, zinc, copper or nickel, aluminum is replaced by iron or chromium, and carbonate is replaced by chloride, bromide, fluoride, nitrate, acetate, cyanate, sulfate, chromate, oxalate, phosphate or ferrocyanate, with the stoichiometry adjusted appropriately, as a catalyst for preparing ethylene glycol by reacting ethylene carbonate and water. The disclosures note that these hydrotalcite-type catalysts are easily separated from the liquid reaction medium and are stable under the hydrolysis reaction conditions (130.degree.-160.degree. C.).
U.S. Pat. No. 4,774,212 to Drezdon and Drezdon, "Synthesis of Isopolymetalate-Pillared Hydrotalcite Via Organic-Anion-Pillared Precursors", Inorganic Chemistry, 27:4628-4632 (1988) relate to synthesis of hydrotalcites for vapor phase dehydrogenation or ammoxidation of hydrocarbons. These hydrotalcites contain polyoxometalates of vanadium, tungsten and molybdenum as the interstitial anions, and are made by substituting such polyoxometalates into hydrotalcite structures that contain large organic anions.
Dimotakis et al., Inorganic Chemistry, 29: 2393 (1990) describes the synthesis of hydrotalcites and other layered double hydroxides containing organic anions using swelling agents such as glycerol.
U.S. Patent No. 4,967,018 to Soo et al. describes a process for the hydrolysis of alkylene oxide to alkylene glycol using hydrotalcite-type mixed-metal framework compositions as catalysts. Soo et al. list a variety of metal cations and interstitial anions that may be used, and disclose that such mixed-metal framework compositions provide particularly high selectivity to monoalkylene glycol when the interstitial anion is selected from metavanadate, orthovanadate, hydrogen pyrovanadate, pyrovanadate, molybdate and tungstate.
Because of the relative desirability of monoalkylene glycol over higher alkylene glycols in the commercial marketplace, it is necessary to employ catalysts in alkylene oxide hydrolysis that are highly selective to monoalkylene glycol. Increases in selectivity to monoalkylene glycol of only a few percentage points translate into large quantities of highly valuable commercial product. Accordingly, the need exists for ways to enhance the selectivity of hydrotalcite-type alkylene oxide hydrolysis catalysts while maintaining catalyst stability (i.e., minimum loss of catalyst constituents such as by leaching). Moreover, since many of the alkylene oxide hydrolysis processes now in commercial use are uncatalyzed, liquid phase processes, it is also desirable for any proposed alkylene oxide hydrolysis catalyst to be retrofittable into current commercial equipment.